Something important that seems to be missing from the website's available information: what happens when you hit the limit on free messages but haven't paid? I

Something important that seems to be missing from the website's available information: what happens when you hit the limit on free messages but haven't paid? It seems like there's two possibilities with any likelihood, either it gives the user an error or it starts behaving like a regular mailto: link until the limit rolls over - but it'd be nice if the site said which.